Method of production of a liquid crystal cell

ABSTRACT

A method of production of a liquid crystal cell, which comprises the steps of covering a first plate with crystals of a crystal fraction. The limits deviate from each other in case of an upper crystal limit of no more than 25 um no more than 6 um and in case of an upper crystal limit of more than 25 um no more than 25% of the upper crystal limit. The crystals are arranged in one layer spaced apart at distances from each other. The distances surpass appreciably at an average the crystal size. A second plate is provided on the first plate covered with the crystals. The arrangement of the steps of providing first and second plates until a predetermined number of plates being obtained is repeated. The pile of the plates is heated with simultaneous application of pressure. The plates are hermetically connected at their edge zones with a provision of filling- and ventilating openings. The plates are then cooled off. The spaces defined between each pair of adjacent plates are filled with said crystal liquid, and the filling- and ventilating openings are hermetically filled.

United States Patent Bayer [4 1 May 27, 1975 METHOD OF PRODUCTION OF ALlQUlD CRYSTAL CELL 21 Appl. No.: 351,526

Related US. Application Data [62] Division of Ser. No. 208,340, Dec. 15.1971.

[30] Foreign Application Priority Data Dec. 21, 1970 Austria 11509/70[52] US. Cl. 156/145; 65/43; 156/109; 156/222; 156/288; 156/311; 350/160LC Int. Cl 1332b 31/20 Field of Search 156/89, 99, 100, 102, 107.156/145, 311, 106, 89,109, 146, 222', 65/43; 53/38. 41, 43; 350/160 LC;52/616; 161/45.

NIKYANR 3,703,329 11/1972 Castellano 350/160 LC Primary Examiner-CharlesE. Van Horn Assistant Examiner-F. Frisenda, Jr,

Attorney, Agent, or FirmErnest G. Montague; Karl F. Ross; Herbert Dubno[57] ABSTRACT A method of production of a liquid crystal cell, whichcomprises the steps of covering a first plate with crystals of a crystalfraction. The limits deviate from each other in case of an upper crystallimit of no more than 25 um no more than 6 um and in case of an uppercrystal limit of more than 25 um no more than 25% of the upper crystallimit. The crystals are arranged in one layer spaced apart at distancesfrom each other. The distances surpass appreciably at an average thecrystal size. A second plate is provided on the first plate covered withthe crystals. The arrange ment of the steps of providing first andsecond plates until a predetermined number of plates being obtained isrepeated. The pile of the plates is heated with simultaneous applicationof pressure. The plates are hermetically connected at their edge zoneswith a provision of fillingand ventilating openings. The plates are thencooled off. The spaces defined between each pair of adjacent plates arefilled with said crystal liquid, and the fillingand ventilating openingsare hermetically filled.

6 Claims, 11 Drawing Figures gamma m 30 35 METHOD OF PRODUCTION OF ALIQUID CRYSTAL CELL This is a divisional patent application to thecopending patent application Ser. No. 208,340, filed Dec. 15, 1971.

The present invention relates to a method of produc tion of a liquidcrystal cell, in which the crystal liquid is embedded between plates ofinorganic material, in particular glass, of which at least one istransparent and which are covered under circumstances with transparentor reflecting, respectively, thin-layer-electrodes, as well as a methodfor its production.

One understands under a liquid crystal cell, a controllable light gateor a controllable reflector, in which the particular characteristicsoccuring between the melting point and the clearing point of crystals,that means, anisotropic, liquids are exploited. In the embodiment as alight gate, all plates and thin-layerelectrodes must be transparent, inthe embodiment as a reflector, one requires in addition to transparentplates and thin-layer-electrodes at least one reflectingthin-layer-electrode, whereby its carrier plate can be non-transparent.

For liquid crystal cells one uses in the practice first of all nematiccrystal liquids and as controlling influence values preferably electricfields. For the practical application it is required, to formcomparatively thin layers of the crystal liquid, the thickness of whichis of the order of l to pm, in particular of 5 to 30 um. Until now onehas proceeded such, that between plates of transparent material,preferably first of all of glass, which have been covered according torequirements with transparent of relecting, respectivelythin-layerelectrodes, strips at the plate edge, or partly interruptedframe of a foil, the thickness of which corresponds with the opendistance. As foil material has been used thereby mainlypolyhexaflourethylene. After the filling of crystal liquid the cell hasbeen closed with an epoxy resin. In this embodiment at some distancefrom the edge the permanency of the distance, in particular in case ofpressure loads, something has been left open and desired. By thisarrangement the production of large face liquid crystal cells is moredifficult, if not impossible. Furthermore, the life duration is modestwith about lO hours, for which, first of all, is named as cause, the noncompletely hermetic closure, which does not prevent sufficiently thefounding-in of solvent vapors and atmospheric elements.

A summary view concerning the characteristics of crystal liquids can befound by example in the publication:

A. Sauper Neuere Ergebnisse auf dem Gebiet der flussigen Kristalle"(Late results in the field of liquid crystals), Zeitschrift furangewandte Chemie, 80th year (1968) pages 99l l5, or in H. Sackmann undD. Demus" Eigenschaften und Strukturen Thermotroper kristallineflussigerZustande, (characteristics and structures of thermotropic cystal liquidstates). Fortshcritte der chemischen forschung, l2th year U969), pages349386. Practical applications are described by example in US. Pat. No.2,400,877, No. 2,524,286, No. 2,544,659, No. 3,l35,207 and No. 3,322,485and in the publication Electronics" of July 6, I970, pages 64 to 70, andin the publication Electronic Design" of Sept. 13, I970, pages 76 to 81.

By the Austrian Pat. No. 284,361, a method for production ofmultiple-glass-units have been known,

whereby the plates and the formation, of a stay at the edge have beenmolten and at least in one corner of the unit a drain hole is providedin the one plate, which is eventually tightly closed. For liquid crystalcells such produced multiple glass units are not usable, since themelting process required for their production would destroy thethin-layer-electrodes and the required narrow open distance cannot beproduced or cannot be produced with the required tolerance,respectively.

By the Austrian Pat. Nos. 26l,0l5 and No. 261,0l6 practically the use ofa core of thick core layers for the production of an electrode systemhas become known, however, where the cores and crystals do not serve fornarrow distancing of plates.

It is one object of the present invention, to provide a method of makinga liquid crystal cell, in which the mentioned drawbacks are avoided andthe create a liquid crystal cell corresponding better the rough practiceconditions.

It is another object of the present invention to provide a method ofmaking a liquid crystal cell, wherein for distancing of the plates atsingle layer of cores or crystals is provided there between, throughwhich the open distance of the plates is determined, whereby the mediumdistance of the crystals is essentially larger, for example, at leastten times as large as the median core size, and the plates arefurthermore, connected together at their edge zones, preferably by hotpress welding or, by means of a glass solder hermetically for thepurpose of filling of the crystal liquid filling and airing openingswhich remain free at first are hermetically closed.

Aside from the fact, that the desired aim is obtained, thereby, nowessentially thinner plates can be used, compared with the knownstructures, which in addition to material and space saving offers thepossibility for advantageous further developments of the present invention, which are set forth below. Also the production of large facedliquid crystals cells is essentially simplified by the presentinvention.

If plates covered with thin layer-electrodes are applied, the crystalsmust be laid as it is understood of insulating material.

While in the practice mainly nematic crystal liquids or mixtures with anematic crystal liquids are applied as a main component for liquidcrystal cells, the present invention is not limited thereto, ratherextends to all sorts and mixtures, as long as they show only anisotropicqualities.

Concerning the narrowness of the crystal spectrum, no extremerequirements are necessarily set. It has been found in the practice assufficient, to use crystal fractions, the limits of which are at anupper crystal limit of more than 25 um no more than 25% of the uppercrystal value and deviate at an upper crystal value of less or equal to25 um no more than 6 uum from each other. Such crystal fractions can beobtained by example by straining with strain fabrics according to DIN 4l88', whereby even one step can be jumped over, as by example crystals of25 um to 32 um open mesh width or from 28 pm to 36 um open mesh width.Also reduction processes are known in which one obtains without sieves anarrow crystal spectrum by example by means of a spray mill, which isapplied in particular at crystal sizes below 25 pm with advantage. Alsoby means of wind sifters one can obtain a usable narrow crystalspectrum.

The great tolerance relative to the crystal size can be permitted forthe reason, that with the connection of the edge zones by means of hotpress welding the crystals, which are pressed into the plates in theedge zones, as much as they are present there, penetrate in theremaining plate range partly into the plates, partly they are pressedslightly flat, whereby, the open distance sets with much narrowertolerance. It is also possible, by suitable selection of the workingmaterial, to aim at only the penetration or for only flat pressing,respectively. In case of a connection of the plates without heating, thenarrow tolerance of the open distance is brought about such, that atfirst only the crystals disposed at the upper crystal limit are tightlyclamped and it is thus possible, to remove the remaining by example byvibration.

The number of the filling-or airing-openings, respec tively, can bereduced to a single opening for each liquid crystal layer, if the airbetween the distanced plates is evacuated prior to the filling of thecrystal liquid, whereby suitably a three-way-cock is used. The closureof the filling or airing-opening can in known manner take place withoutdisadvantages by means of an epoxy resin, since the face, which engagesthe crystal liquid, is appreciably smaller than in the conventionalembodiments and this engagement takes place in addition at a dead arm."The closure can take place however, also by welding or melting, wherebyagain the dead arm" protects the main quantity of the crystal liquid.

Since the cores cover concerning the face at a maximum of about I% it isnow ofimportance for many purposes of application, by example, for bylight writing, if the cores consist of non-transparent material. Onecan, however, if necessary bring to disappearance optically sufficient,if they consist of a transparent material, which has at leastapproximately the same defraction index, as the crystal liquid. It isfurthermore possible to hold free from cores without essential loss onrigidity, individual, no too large areas of the liquid crystal cell. Inthe recess of these areas, one uses suitably masks, sieve pressuregages, or the like.

Due to the fact, that now essentially thinner plates can be used withoutimpairing the stability, it is easier possible, to form liquid crystalcells in form of more layers, that means, a plurality of plates areprovided forming a pile, whereby in the intermediate spaces between theplates, crystal liquid is arranged. These separate layers of crystalliquid can advantageously consist of different crystal liquids, or aremade with different crystal liquids in which foreign matter has beenincluded. The configuration of the thin layer-electrodes can thereby bedifferent from layer to layer. Also the production of curved liquidcrystal cells is possible in a simple manner in connection with theembodiment of the pzesent invention. Since, namely the plate pile isdistanced over its surface by means of the cores or crys tals after theconnection of the plates by heating and following cooling it can becurved in a suitable form, without losing the exact distancing.

In accordance with a further development of the present invention, theliquid cell is formed as an image screen by example, for an oscillator,or for a television receiving device, whereby a device known per se forthe production of the image screen is provided. For the formation as animage screen of a color television receiving device suitably as manylayers of crystal liquid equipped with a liquid dotted withcorresponding color material are provided, as basic colors are used forthe creation of the color picture.

For the production of a liquid crystal cell, in accordance with thepresent invention, one provides suitably an arrangement, wherein thefirst plate is covered with cores of one core fraction, the limits ofwhich deviate from each other at an upper core limit of less or equal to25 am no more than 6 pm and at an upper core limit of more than 25 um nomore than 25% of the upper core limit, whereby the cores by means of asieve or by spreading of a proportioned quantity sieve pressure gages,vibration distribution and the like are disposed in one layer spacedapart from each other, which surpass at an average the core sizeappreciably, onto which plate covered with cores the second plate isdisposed, these method steps are continued up to the reaching of thedesired number of plates, whereby the total pile is heated up withexertion of pressure, whereby at the edge zones with the provision ofgaps of filling-or airing-openings are hermetically connected andfinally brought to cooling down, whereupon the crystal liquid is filledin and the filling-or airingopenings, respectively, are hermeticallyclosed.

Upon presence ofa 15 hours stress releaving temperature (strain point)of the core material lower in comparison with the plate materialadvantageousiy the total pile is heated up to a temperature, whichsurpasses the 15 hour-stress releaving temperature (strain-point) of thecore material and the IS hour-stress releaving temperature of the platematerial is not reached, whereby the cores are pressed together to theopen distance to be produced and adhere by engagement of the face to theplates or foreign layers by engagement of their faces.

Upon presence of a 15 hour stress releaving temperature (strain-point)of the core material compared with the plate material equal or higher incomparison with the plate material suitably the total pile is heated toa temperature which surpasses the 15 hour stress relieve temperature(strain-point) of the plate material. The plate can thereby be pressedand connected together over parts of their edges during the heating (hotpress welding).

The production method in accordance with the present invention ofiersfirst of all the advantage, to be capable of rationalization with acomparatively low expenditure.

With these and other objects in view, which will become apparent in thefollowing detailed description, the present invention which is shown byexample only, will be clearly understood in connection with the threeembodiments, and in connection with the accompanying drawings, in which:

FIG. 1 is a section along the lines [-1 of FIG. 2;

FIG. 2 is a top plan view of the liquid crystal cell, designed inaccordance with the present invention;

FIG. 3 is a perspective view shown in an enlarged scale by a partlybroken upper portion of the liquid crystal cell;

FIG. 4 is a fragmentary view of the liquid crystal cell shown at anenlarged scale;

FIG. 5 is another embodiment of the fragmentary showing in FIG. 4',

FIGS. 6 to 9 show top plan views of different embodiments of thin layerelectrodes;

FIG. 10 is a section through a liquid crystal cell having a plurality oflayers; and

FIG. II is a section through a curved liquid crystal cell.

Referring now to the drawings and in particular to FIGS. 1 to 5, aliquid crystal cell is disclosed which indicates the basic principle ofthe present invention. The liquid crystal cell comprises two glassplates 1 and 2 covered practically totally on the inside with thinlayerelectrodes 11 and 12, between which glass plates 1 and 2 a singlelayer of cores 3 of insulating material is disposed, through which theopen space of the plates is determined. The cores or crystals 3 arethereby, for the sake of clearness and better showing indicated in thedrawing, shown of a greater height. At their edge zones 4, 5, 6 and 7the glass plates are hermetically connected together by hot pressingwelding, whereby it is to be understood as the application of heat andpressure. At two corners filling-or-air-openings, respectively, 8 and 9are recessed by providing, by example, during the hot press welding atthe place of the openings 8 and 9 to be produced, wires of a materialwith greater thermic expansion coefficients compared with the materialof glass plates, as about copper, which wires can be easily removedafter cooling off whereby the desired openings 8 and 9 are created.Through one of these openings 8 a nematic crystal liquid 10 or a mixturewith a nematic crystal liquid as main component is filled in, whilethrough the other opening 9 the air can escape. The openings 8 and 9 areclosed up by means of epoxy resin 13 and I4, whereby only very smallengaging faces of the epoxy resin 13 and 14 with the crystal liquid l0and in particular at a dead arm," occur. The number of the openings 8and 9 is, as a matter of course, not limited to two, nor must they be atthe corner diagonally opposite each other, the shown embodiment has beenfound as particularly practical. The closure of the openings 8 and 9 cantake place also by welding or melting. Upon application of a voltage tothe thin layer-electrodes l1 and 12, the previously open transparentnematic crystal liquid 10 becomes in known manner opaque, whereby thedegree of the shading depends upon the magnetizing force. For thisreason, it is of essence, to maintain the distance of the plates 1 and 2exactly over the entire surface, for which, in accordance with thepresent invention, the cores 3 serve. The control can also take place bymeans of a magnetic field, in which case the thin layerelectrodes 11 and12 are dispensible.

If one choses for the cores 3 a transparent working material, which hasat least approximately the same refraction index as the crystal liquid10, the cores become practically invisible, for which reason they arenot seen in FIG. 2.

In order to show clearly the principle of the present invention, asection of the liquid crystal cell is shown enlarged in a perspectivefragmentary view in FIG. 3. One portion of the glass plate I and thethin layerelectrode II have been broken away. The distance between theplates 1 and 2 amounts in reality 10 to IO pm, in a realized example pmthe thickness of the thin plates 1 11 and 12 lies at the order of 0.5pm. It is apparent, that the medium distance of the cores 3 surpassesthe core size appreciably, about at least 10 times, wher' y, the spacerequired by the cores 3 is at a maximum of about I% of the spaceremaining from the crystal liquid 10. Though the cores 3 thus take onlyvery little space and can also be brought optically to disappearance,they effect a decisive improvement of the rigidity of the cell and ofthe uniform distancing of the glass platesl and 2 over the face. Thethickness of the glass plates I and 2 can thereby be reduced comparedwith known structures, which enlarges the possibility of use of liquidcrystal cells. A further advantage of the present invention resides inthe fact, that no more such high requirements must be set to the surfacequality of the glass plates 1 and 2, as in the known embodiments.

Referring now again to the drawings and in cores to FIGS. 4 and 5, theseFigures show a strongly enlarged section of FIG. 1 and in particular twoembodiments of the distancing of the glass plates 1 and 2 by the cores3. In the embodiment according to FIG. 4 the core working materialsoftens before the plate working material, while in the embodimentaccording to FIG. 5 the plate working material softens before the coreworking material. These two embodiments are to be understood as extremesbetween which flowing transitions exist. If by example the glass plates1 and 2 and the dore 3 are of the same glass, a not so stronglypenetration of the cores 3 into the glass plate 2 similar to FIG. 5results, because the temperature of the cores 3 will remain back duringthe heating process relative to the glass plates 1 and 2. The lastembodiment offers the best rigidity results. The cores 3 can therebyengage the thin layer-electrodes l1 and 12, as shown in FIGS. 4 and 5 inconnection with the plate 2 (thin-layer-clectrode 12), whereby it canpress in the thin layer-electrode 12 also somewhat into the plate 9(FIG. 5). They can separate, however, the thin layer-electrodes 11 and12, as is shown in plate 1 (thin layer-electrode ll).

Since the liquid crystal cells, designed in accordance with the presentinvention, are designed robust of chemically and termically permanentworking mate rial, it is suitable without further protection andmeasures for crystal liquids of the most different types, thus inaddition such with the anisotropic range at room temperature also forsuch with higher and lower disposed anisotropic range.

As stated, in the embodiment of a liquid crystal cell controllable byelectric fields, it is necessary to cover the glass plates 1 and 2 inknown manner at the inside with thin layer-electrodes 11 and 12. Suchtransparent thin layer-electrodes 11 and 12 are produced by example oftin oxide or indium oxide and reflecting, by example, of aluminum. Ifone covers the entire plate face with thin layer-electrodes I1 and 12,one obtains an electrically controllable light gate or an electricallycontrollable reflector of the simplest type, respectively. At the edgezones 4 and 6 only one thin layer-electrode II and 12 is guidedoutwardly, as by example, in FIG. 3 for the thin layerelectrode I2 isshown at the edge zone 6. At the edge zones 5 and 7 small thinlayerelectrodes 11 and 12 are applied.

By formation of the thin layer-electrodes ll and 12 in the mostdifferent configuration, it is knowingly possible to produce controldifferent signs, by example, numbers and letters.

FIGS. 6 to 9 show known embodiments therefor. The glass plate 15 shownin FIG. 6 in elevation carries, by example, a thin layer electrodeconfiguration I6, 17, I8, 19, 20, 21 and 22, as it is conventional for asieve segment number indication display. The glass plate 23 shown inFIG. 7 is likewise covered by a thin layerelectrode 24, which can serveas counter electrode for the one shown in FIG. 6.

FIGS. 8 and 9 show further examples for practically often used thinlayer-electrode configurations. The glass plate 25 shown in FIG. 8 hasvertically extending paths of thin layer-electrodes 26a to 263, whilethe glass plate 27, shown in FIG. 9, has horizontally extending paths ofthin layer electrodes 28a to 28g. If one forms with the glass platescovered with the paths of thin layer-electrodes 26, 28 a liquid crystalcell in accordance with FIGS. 1 and 2, one obtains the known screen,whereby only that small range point" comes to lighting, the horizontaland vertical electrodes 26 and 28 of which is applied to voltage.

FIG. shows a section through a liquid crystal cell in accordance withthe present invention having a plurality of layers and in particularthree layers. Four glass plates 29, 30, 31 and 32 are spaced apart fromeach other by one layer of cores 3 and connected together at the edgezones 33 and 34 by means of glass solder 35. In order to fill crystalliquid into the intermediate spaces suitably similar to FIG. 2 at twooppositely disposed corners for each layer 36, 37 and 38, two fillingandairingopenings 8 and 9 are provided, whereby the openings 8 and 9 of themediate layer is set off diagonally advantageously relative to those ofthe first and third layers. The openings can be produced by omitting ofa split during application of the solder glass quantity, which takesplace suitably by means of sieve pressure. Each layer 36, 37 and 38 ofcrystal liquid is specially completely separated from the remainingportions Furthermore. it is possible to provide transparent thinlayer-electrodes insulated from each other at the upper side and thebottom side of the glass plates 30 and 31, the glass plates 39 and 32require merely on their inner side thin layer-electrodes, so that alsoin relation to their electrical control a complete separation ispossible. Thus it is apparent that each layer 36, 37 and 38 of crystalliquid is completely independent from the remaining portion and cantherefore also consist of different crystal liquids or dotted withdifferent foreign matter by example coloring material. The practicalapplication ofa multi layer liquid crystal cell is essentiallysimplified by the possibility, according to which in accordance with thepresent invention the glass plates 29, 30, 3l and 32 are formed thinwithout loss of robust ness, if not made possible at all.

For the formation of the liquid crystal cell. in accordance with thepresent invention, as an image screen, the glass plates have in knownmanner, similar to the glass plates 25 and 27 in FIGS. 8 and 9, thinlayerelectrodes 26 and 28. An image screen for a color televisionreceiving device is suitably constructed similar to FIG. 10, whereby thethin layer-electrodes are disposed similar to FIGS. 8 and 9.

In accordance with the present invention it is now possible withoutgreat expenditure to produce curved crystal liquid cells. FIG. ll showsa section through a curved liquid crystal cell. It is at first exactlyconstructed as the crystal cell shown in FIGS. 1 and 2. After orsimultaneously with the hot press welding the glass plates 39 and 40spaced apart already by the cores 3 are brought in a form of a workingmaterial which is not wetted by glass, by example, graphite or boronnitrate, whereupon by heating in the transformation range of the glassused for the glass plates 39 and 40, the glass plates deform in desiredmanner, which form is retained after cooling off. Only then the crystalliquid 10 is filled in.

While I have disclosed several embodiments of the present invention, itis to be understood, that these embodiments are given by example onlyand not in a limiting sense.

I claim:

I. A method of producing a liquid-crystal cell comprising the steps of:

a. distributing substantially uniformly over a surface of a first plategrains of a particle size deviating by no more than 6 microns for grainsof a particle-size range with an upper limit of 25 microns and deviatingby no more than 25% of the upper particle-size limit for grains ofaparticle size range of more than 25 microns;

b. spacing the grains distributed on said first plate in a single layerwith an intergrain spacing appreciably exceeding on the average, thegrain size;

c. applying a second plate to said layer of grains on said first plateand covering said second plate with a second layer of grains;

d. repeating steps (a) (c) to form a stack of plates with respectivesingle layers of grains between suc cessive plates to define respectivecompartments until the stack attains a predetermined number of plates;

e. heating and applying pressure to said stack of plates to bond theplates of said stack to said grains together;

f. hermetically sealing the pairs of plates defining each compartmenttogether at their respective edge zones while leaving filling andventing openings therealong;

g. cooling said plates;

h. filling each of said compartments with a liquidcrystal material; and

i, hermetically closing said openings.

2. The method defined in claim 1 wherein said grains have aIS-hOur-stress-reIieving temperature which is less than that of saidplates, said stack being heated to a temperature exceeding thel5-hour-stress-relieving temperature of said grains but below the15-hourstress-relieving temperature of said plates, said plates beingpressed together to define the interplate width of each compartment.

3. The method defined in claim I wherein the plates are hermeticallyconnected along their edges at least in part by glass solder.

4. The method defined in claim 3 wherein said glass solder is applied byscreening onto said plates.

5. The method defined in claim 1 wherein said plates have al5-hour-stress-relieving temperature below that of said grains, saidmethod comprising heating said stack of a temperature above thel5-hour-stress relieving temperature of said plates but below that ofsaid grains.

6. The method defined in claim I further comprising bending said stackduring the heating thereof.

1. A method of producing a liquid-crystal cell comprising the steps of: a. distributing substantially uniformly over a surface of a first plate grains of a particle size deviating by no more than 6 microns for grains of a particle-size range with an upper limit of 25 microns and deviating by no more than 25% of the upper particle-size limit for grains of a particle size range of more than 25 microns; b. spacing the grains distributed on said first plate in a single layer with an intergrain spacing appreciably exceeding on the average, the grain size; c. applying a second plate to said layer of grains on said first plate and covering said second plate with a second layer of grains; d. repeating steps (a) - (c) to form a stack of plates with respective single layers of grains between successive plates to define respective compartments until the stack attains a predetermined number of plates; e. heating and applying pressure to said stack of plates to Bond the plates of said stack to said grains together; f. hermetically sealing the pairs of plates defining each compartment together at their respective edge zones while leaving filling and venting openings therealong; g. cooling said plates; h. filling each of said compartments with a liquid-crystal material; and i. hermetically closing said openings.
 2. The method defined in claim 1 wherein said grains have a 15-hour-stress-relieving temperature which is less than that of said plates, said stack being heated to a temperature exceeding the 15-hour-stress-relieving temperature of said grains but below the 15-hour-stress-relieving temperature of said plates, said plates being pressed together to define the interplate width of each compartment.
 3. The method defined in claim 1 wherein the plates are hermetically connected along their edges at least in part by glass solder.
 4. The method defined in claim 3 wherein said glass solder is applied by screening onto said plates.
 5. The method defined in claim 1 wherein said plates have a 15-hour-stress-relieving temperature below that of said grains, said method comprising heating said stack of a temperature above the 15-hour-stress-relieving temperature of said plates but below that of said grains.
 6. The method defined in claim 1 further comprising bending said stack during the heating thereof. 